1. Field of the Invention
The present invention relates to a method for the reduction of metal sulfates. More particularly, the invention relates to a process for improving the reducing potential of natural gas used in the reduction of metal sulfates such as gypsum.
2. Background
A wide variety of processes have been developed for the processing of metal sulfates such as gypsum (CaSO.sub.4). These processes often focus on the recovery of lime (CaO) and a form of sulfur, typically either elemental sulfur (S) or sulfur dioxide (SO.sub.2) with the later often used for the production of sulfuric acid. These processes typically involve the reduction of the metal sulfate to form the metal sulfide as the first step followed by a second oxidation step for conversion of the sulfide to sulfur and/or sulfur dioxide.
The reductive decomposition of calcium sulfate is more fully illustrated by the following equations: EQU CaSO.sub.4 +CO.fwdarw.CaO+CO.sub.2 +SO.sub.2 (1) EQU CaSO.sub.4 +H.sub.2 .fwdarw.CaO+H.sub.2 O+SO.sub.2 (2) EQU CaSO.sub.4 +4CO.fwdarw.CaS+4CO.sub.2 (3) EQU CaSO.sub.4 +4H.sub.2 .fwdarw.CaS+4H.sub.2 O (4)
Equations (1) and (2) are carried out under what is referred to as mildly reducing conditions, that is, a relatively low amount of reducing agent such as carbon monoxide or hydrogen or both. Reactions (1) and (2) require heat, i.e., are endothermic, and are favored by higher reaction temperatures. Reactions (3) and (4) are carried out under strongly reducing conditions, that is, a high amount of reducing agent such as carbon monoxide or hydrogen or both. Reactions (3) and (4) give off heat, i.e., are exothermic, and are favored by lower reaction temperatures.
There have been a large number of efforts to maximize the efficiencies of sulfate reduction including use of either mild (equations (1) and (2)) or strong reducing conditions (equations (3) and (4)). These efforts have been directed to achieving the proper reaction conditions for the selected reaction equations. One of the areas of focus has been in the area of reducing agents used to reduce the sulfate.
A wide variety of reducing gases have been used in a variety of reaction schemes to achieve this result. For example, Foecking, U.S. Pat. No. 3,607,036, relies on the use of a reducing gas and steam to reduce calcium sulfate to the sulfide which in turn reacts with water to form hydrogen sulfide which is also used as a reducing gas for the reduction of the sulfate. The Foecking reducing gases include hydrogen, carbon monoxide, hydrocarbons such as methane, natural gas, and primary reform gas. He obtains primary reform gas by heating water and a gas such as methane to give a composition of about 15 percent carbon monoxide, 6-8 percent carbon dioxide, 0.2-0.3 percent methane, with the remainder being hydrogen. Temperatures in the range of 600.degree.-900.degree. C. (1112.degree.-1652.degree. F.) are used with a temperature of 800.degree.-850.degree. C. (1472.degree.-1562.degree. F.), preferred.
Campbell, U.S. Pat. No. 3,582,276, is directed to obtaining optimal amounts of sulfur dioxide in the reduction step and thus uses a high-temperature environment (2000.degree.-2500.degree. F.; 1093.degree.-1371.degree. C.) and low amounts of reducing gas to favor equations (1) and (2), supra. Campbell uses hydrogen, or carbon monoxide or mixtures of the two such as obtained from the steam reforming of natural gas. He also describes the partial oxidation of natural gas with air as the source of a suitable reducing gas mixture. The reducing gas is prepared in a partial oxidation furnace using air and natural gas to obtain a reducing gas mixture at a temperature of 2,500.degree. F. (1371.degree. C.). Such a temperature favors the formation of calcium oxide and sulfur dioxide in equations (1) and (2), supra.
U.S. Pat. No. 4,102,989 and U.S. Pat. No. 3,607,045 to Wheelock produces calcium oxide and sulfur dioxide from gypsum by simultaneously carrying out both oxidation and reduction actions in the same bed and teaches the in situ combustion of fuels such as coal and natural gas in the bed to produce a reducing environment. Wheelock uses a high temperature environment 1950.degree.-2250.degree. F. (1066.degree.-1232.degree. C.) to favor the reactions in equations (1) and (2). To maximize the heating efficiency of reducing agents such as natural gas, the apparatus is arranged so that separate flows of air and natural gas are maintained to the reaction bed to avoid precombustion.
Orahood, U.S. Pat. No. 3,661,518, is concerned with the recovery of calcium sulfide, itself. To achieve this result, he uses the strongly reducing environment of equations (3) and (4). A reducing medium of reformed methane, i.e., carbon monoxide and hydrogen, and temperatures of 1800.degree. F. (982.degree. C.) are used. Howard et al, U.S. Pat. No. 1,457,436, is concerned with the reduction of metal sulfates to sulfides by burning carbonaceous materials, liquid fuels or a combustible gas in the reaction chamber to produce reducing conditions.
Moss, U.S. Pat. No. 4,041,141, recovers sulfur from sulfur-containing solid fuels, such as coal, by reacting calcium oxide with the sulfur-containing fuels to obtain calcium sulfate and sulfide. He burns natural gas and a limited amount of air in the reactor bed to form mildly reducing conditions according to equations (1) and (2) for the conversion of calcium sulfate to calcium oxide and sulfur dioxide. The reactor bed is about 900.degree.-1350.degree. C. (1652.degree.- 2462.degree. F.) with a temperature of about 1050.degree.-1090.degree. C. (1922.degree.-1994.degree. F.) preferred. The calcium oxide is recycled for further reaction with the sulfur containing fuels.
In summary, it is seen that natural gas has been used under a variety of conditions for sulfate reduction. Excess water has been used with natural gas to liberate hydrogen sulfide from calcium sulfide. Natural gas has been partially burnt to form a high temperature reducing agent that is fed to the sulfate reactor to give the high temperature and mildly reducing conditions of equations (1) and (2). Natural gas also has been partially burnt within the reactor bed to give the high temperatures that promote reaction (1) and (2) through the production of both heat and a reducing environment in the reactor bed. Natural gas has been heated with water to produce reform gas that is used to afford the lower temperatures and stronger reducing conditions of equations (3) and (4). However, no attempt has been made to improve the reducing potential of natural gas itself.